Salinity resistance as a function of NH4+:NO3- ratio and its impact on yield and quality of tomato (Solanum lycopersicum L.)

Background: Adequate regulation of plant nutrients may constitute a promising approach to overcome the deleterious effects of salinity on plant growth and development. Among these nutrients, nitrogen (N) can be a stand out option to improve plant adaptation to saline environment. However, plant response to N under salinity stress may vary depending upon NH4+:NO3- ratio in the growth medium. Aims: The present study aimed to determine the efficiency of different NH4+:NO3- ratios for improving salt resistance of tomato under alkaline conditions. Methods: Experimental plan comprised of five NH4+:NO3- ratios 100:0, 75:25, 50:50, 25:75 and 0:100, and three salinity levels (no salinity, moderate salinity of 7 dS m(-1), and high salinity of 10 dS m(-1)), with five replications. Plants were grown up to maturity and fruits were harvested for quality analysis. Results: NH4+:NO3- ratios produced a differential response in ameliorating salinity toxicity. Plant growth and fruit yield characteristics reduced with salinity but the extent of reduction varied significantly with NH4+:NO3- ratio under both salinity levels compared to respective NH4+:NO3- ratio without salinity. Physiological characteristics viz. chlorophyll, photosynthesis, malondialdehyde, glutathione, electrolyte leakage and relative water content were also got optimized in favor of the high plant growth and yield by NH4+:NO3- ratio of 50:50 under salinity stress. Among different NH4+:NO3- ratios, 50:50 had greater potential not only to reduce Na+ and Cl- but also to promote N, K+, and Ca2+ accumulation in roots, shoots, and fruits, and thus gave more protection against the deleterious effects of salinity. Fruit quality characteristics improved with salinity, except for vitamin C. Shoot N correlated positively with photosynthesis (R-2 = 0.94) and fruit yield (R-2 = 0.98). Conclusions: Overall, NH4+:NO3- ratios ranked in order of 50:50 > 0:100 > 25:75 > 75:25 > 100:0 to regulate the growth, yield and physiological characteristics of tomato under salinity stress, however best fruit quality characteristics were recorded in 100:0 NH4+:NO3- ratio.